Perhaps the user account with which you installed the program is not the one with which you run it
Or you had a previous installation and the mp3tag.cfg file is corrupt - what happens if you rename it?

But the the light went away while I was using the computer. And I think the app was opened

After such an incident I would always check the file system integrity, e.g. with an administrator command shell and the command
chkdsk c: /v /f
and repeat that for all internal drives.
It may be necessary to boot the system to start the repair process.